Today, petroleum such as oil and gas is oftentimes collected from offshore wells. The collected petroleum must be transported to the mainland for ultimate processing and consumption. This transportation may be accomplished by either loading the petroleum aboard tanker vessels or pumping it through submerged pipelines laid upon the seabed. It has been found that the method of transporting petroleum through pipelines is less expensive, more efficient and safer than by tanker vessels.
Prior to 1991 pipelines were oftentimes not buried and thus exposed to hazards such as the dragging anchors of ships. Occasionally, submerged pipelines also become crimped, broken or dangerously deteriorated and thus must be recovered from the sea floor. Also, unburied pipelines are required to be removed from the sea floor shortly after the operational closing of a well or other production stations.
To date, several methods have been employed to recover submerged pipelines. These methods, as described more fully herein, have proved to be costly and/or dangerous. One method of recovering pipelines has involved having divers transported to the sea floor where they then cut the pipeline into sections. The cut sections are then lifted by a crane to a recovery vessel. Because of the limitations upon a diver, this method of recovery may only be accomplished in relatively shallow waters. However, even in shallow waters this method is still dangerous to divers, time consuming and dangerous to the environment as contaminates within the pipeline may be released to the sea. Beyond a diver's depth limitation submersible vehicles may be used which manipulate cutting equipment. However, vehicular operation is even more expensive and time consuming than that previously described and still has the associated problem of releasing contaminates into the water.
As shown in U.S. Pat. No. 3,971,228, another method of recovering pipeline has been to lift the end of a pipeline to the surface of the water alongside a marine vessel. A pair of cranes then support the pipeline while a section of the pipeline is severed. The severed section is then lifted to the deck of the marine vessel. This method of recovering pipeline while significantly decreasing the risk to a diver nevertheless is also time consuming. Additionally, this method is extremely difficult to implement in other than ideal conditions, for the rising and falling of the vessel causes difficulties in holding and cutting the pipeline.
Pipelines have also been winched onto a barge where they are then cut with a torch. This method of retrieving pipelines has associated problems as well. One problem associated with this method has been with regard to the cutting of the pipeline with a torch. As these pipelines have been used to transport oil and gas they oftentimes, even after flushing, contain hydrocarbons which vaporize over time causing the hydrocarbons to be ignited with the torch. Another problem has been the safety to personnel as the pipeline rests upon the deck of the barge during this procedure, for the barge may drift thereby causing the shifting of the pipeline on board or the pipeline may spring or roll upon cutting. Thus, this method while increasing safety by limiting personnel time underwater creates safety problems aboard the vessel.
It thus is seen that a need remains for a system to recover marine pipeline and a method of recovering such in a safe and efficient manner. It is to the provision of such that the present invention is primarily directed.